In recent years, the demand for a battery as a power supply for portable devices has been increasing with the development of the portable devices such as a personal computer and a mobile phone. Such an application requires the battery to be used at a room-temperature and to have a large energy density and superior cycling characteristics.
For such a demand, nonaqueous electrolyte lithium batteries employing lithium ions as charge-transfer media have been developed using various types of nonaqueous electrolytes. The nonaqueous electrolytes include, for example, organic electrolyte, gel polymer electrolyte in which organic electrolyte is non-fluidized using polymer or gelling agents, and solid electrolyte. Further, materials with a high reversible electric potential for reversibly storing and releasing lithium ions from/to various types of electrolytes are discovered, and are used as positive electrode active material. These materials include, for example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), and lithium manganate (LiMn2O4). Meanwhile, elementary substances, alloys, or compounds with a low reversible electric potential, such as graphite or various types of carbon material, are discovered, and are used as negative electrode active material. Furthermore, a lithium battery has been developed that employs these materials for storing and releasing lithium ions as active materials.
As the function of the portable devices has been improved, the power supply has been required to have energy density larger than ever. This requirement is intended to be satisfied by increasing the charge voltage per single cell of battery. In this case, oxidative decomposition of the electrolyte becomes a problem. When charge and discharge are repeated, by-products are deposited on an interface between the positive electrode and the electrolyte, and hence the battery performance is reduced, disadvantageously.
In order to address the problem, Japanese Patent Unexamined Publication No. 2003-338321, for example, discloses a technology of suppressing the oxidative decomposition of the electrolyte by previously forming a film of inorganic solid electrolyte between a positive electrode material and an electrolyte. An example of the inorganic solid electrolyte is lithium phosphate (Li3PO4) or lithium phosphorus oxynitride (LIPON). Such a structure can suppress degradation reaction of the electrolyte, and can keep the battery characteristics even when the charge and discharge are repeated or the charge voltage is increased.
Electrolyte generally has a minuscule amount of residual moisture at 10 ppm level that cannot be removed easily. Here, when above-mentioned Li3PO4 and LIPON come into contact with the moisture—even if their amounts are very small—, phosphorus (P) originally existing as positive pentavalent is reduced to phosphorus with a small oxidation number. Consequently, Li3PO4 and LIPON are decomposed, significantly decreasing the ion conductivity. As a result, side reaction occurs on the interface between the positive electrode and the electrolyte which has been suppressed by the existence of the inorganic solid electrolyte, gas (CO2 or the like) is generated by decomposition of the electrolyte, or by-products (lithium alkoxide or phosphate) after the decomposition are deposited. Therefore, the battery performance is decreased.